1. Field of the Invention
It is widely known that many heart attacks originate from blockages created by athrosclerosis which is the aggressive accumulation of plaques in the coronary arteries. The accumulation of lipids in the artery and resulting tissue reaction cause a narrowing of the affected artery which can result in angina, coronary occlusion and even cardiac death.
Relatively recent studies have shown that coronary disease can also be caused by so-called vulnerable plaques which, unlike occlusive plaques, are engrained or imbedded in the arterial wall and do not grow into the blood vessel. Rather, they tend to erode creating a raw tissue surface that forms caps or scabs. Thus, they are more dangerous than occluding plaque which usually give a warning to a patient in the form of pain or shortness of breath. See, e.g., The Coming of Age of Vulnerable Plaque, Diller, W., Windover's Review of Emerging Medical Ventures, November 2000.
2. Description of the Prior Art
Since vulnerable plaques are contained within the vessel wall, they may not result in a closing or narrowing of that vessel. As a result, such plaques are not easily detectable using conventional x-ray, ultrasound and MRI imaging techniques.
Moreover, since vulnerable plaques are part of the vessel wall, they may have essentially the same temperature as the surrounding normal tissue and the blood flowing through the vessel. Therefore, they are not amenable to detection by known intravascular catheters which rely on infrared (IR) imaging, thermisters, thermocouples and the like in order to detect temperature differences in the vessel wall.
Such intravascular heat sensing catheters are disadvantaged also because they usually incorporate an inflatable balloon to isolate the working each end of the catheter from fluids in the vessel; see for example U.S. Pat. No. 6,475,159. As seen there, the IR detector is located within the balloon which constitutes an insulating (not transparent at IR frequencies) layer between the detector and the vessel wall causing significant attenuation of the signal from the detector. Also, the undesirable stoppage of blood flow by the balloon increases the risk to the patient. Still further, the balloon has to be repeatedly inflated and deflated in order to image different locations along the blood vessel increasing the operating time during which the patient is at risk.
In my above-identified patent application, an intravascular catheter is described which contains a microwave antenna and is able to pick up the presence of vulnerable plaques engrained in the wall of a suspect blood vessel as it is moved along that vessel. The antenna, in combination with an external microwave detection and display unit, detects and displays thermal anomalies due to the difference in the thermal emissivity (brightness) of vulnerable plaques as compared to normal tissue even though the two may have a common temperature. In other words, it has been found that the microwave characteristics of vulnerable plaques imbedded in a vessel wall are different from those of normal tissue comprising the vessel wall and this difference is detected as a thermal anomaly and displayed or plotted as the catheter is moved along the vessel.
In my prior application, the microwave antenna is located at the distal or working end of the catheter. The inner and outer conductors of the antenna are connected by a coaxial cable to the external detection and display unit which includes a radiometer that detects the microwave emissions from the blood vessel picked up by the antenna. The radiometer produces corresponding output signals for a display which responds to those signals by displaying the thermal emissions from the blood vessel in real time as the catheter is moved along the vessel.
The radiometer is preferably a Dicke-type radiometer and a temperature reference, reflecting the temperature of the blood flowing through the vessel which corresponds to the body's core temperature, is used as the radiometer reference. The operating frequency of the radiometer is selected to detect microwave emissions from a depth in the blood vessel wall where vulnerable plaques are likely to be imbedded. Thus as the catheter is moved along the vessel, it is maintained at a constant background or core temperature corresponding to the temperature of the blood and of normal tissue. The locations of vulnerable plaques are detected as thermal anomalies (hot spots) due to the higher emissivity of the plaques as compared to normal tissue. Using the output of the radiometer to control a display, the plaque sites along the vessel can be plotted.
While the apparatus embodiments disclosed in the above parent application are satisfactory in many respects, they have certain drawbacks. More particularly, the Dicke-type radiometer is a comparison radiometer system. Therefore, it requires a Dicke switch to alternately connect the antenna (the unknown temperature to be detected) and the reference temperature, e.g. a stable noise source or temperature sensor.
Every component of such a radiometer generates noise power that contributes to the overall noise of the system. Therefore the total apparatus output contains not only noise received by the antenna, but also noise generated within the apparatus itself. Such variations within the apparatus can produce output fluctuations far greater than the signal level to be measured. To overcome these gain variations, Dicke developed the common load comparison radiometer. This configuration greatly reduces the effects of short-term gain fluctuations in the radiometer since the switch provides a mechanism to allow both the reference and the unknown signals to pass through the apparatus essentially at the same time relative to expected gain drift in the radiometer's amplifiers such that any drifting gain will be applied equally to both the antenna and the reference signals.
Since radiometer receiver input is switched at a constant rate by the Dicke switch between the antenna and the constant-temperature reference load, the switched or modulated RF signal should, therefore, be inserted at a point prior to RF amplification in the radiometer and as close to the antenna as possible. Any component or transmission line located between the unknown temperature being detected and the Dicke switch can introduce an error, i.e. due to changes in environment, motion, etc.
In the apparatus disclosed in my above-identified application, a temperature reference in the form of a resistive load may be provided at the distal end of the catheter to provide a signal corresponding to the patient's core temperature. That approach, however, requires an antenna with a cable-within-a-cable or tri-axial configuration as well as a quarter-wave stub diplexer, to deliver these signals to the radiometer which is located in the external detection and display unit. The inner conductor of the coaxial cable, although in the same environment and subject to the same flexing, is smaller in diameter and therefore not identical to the cable's outer conductor(s). This produces a difference in the gain of the antenna signal verses the reference signal and thus introduces an error into the output of the apparatus as a whole.
Also as noted above, the antennas described in my parent application require a quarter-wave stub diplexer to couple the antenna and reference signals to the external radiometer. This complicates, and increases the cost of, the apparatus disclosed there.
While we will describe the subject apparatus in the context of plaque detection, it should be understood that it may also be use in other applications such as tumor detection, non-invasive temperature monitoring, etc.